Definition - A unit of poetic meter consisting of an unstressed syllable followed by a stressed syllable.

1. It is the most frequently used foot in English meter.

Example -

(1) The phrase da - 'dum is an iamb.
(2) Shakespeare's sonnet Shall I compare thee to a summer's day? is written in iambic pentameter.

Etymology -

Oxford English Dictionary -
Its first citation in this sense is from 1575:
"In the nexte seate to thes hexameters, adonickes, and iambicks, I sett those that stand uppon the number, not in meter, such as my lorde of Surrey is sayde first to have putt forthe in prynte."
(G. Harvey Letter-bk. (Camden) 100 )

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